Half a century after the Doomsday Battle, the uneasy balance of Dark Forest Deterrence keeps the Trisolaran invaders at bay. Earth enjoys unprecedented prosperity due to the infusion of Trisolaran knowledge, and, with human science advancing and the Trisolarans adopting Earth culture, it seems that the two civilizations can coexist peacefully as equals, without the terrible threat of mutually assured annihilation. But peace has made humanity complacent.

The Trisolarian fleet has left their home world and will arrive in four centuries' time. But the Sophons and their extradimensional emissaries are already here. This is the motivation for the Wallfacer Project, a last-ditch defence that grants four individuals almost absolute power to design secret strategies, hidden through deceit and misdirection. Luo Ji, an unambitious Chinese astronomer, is baffled by his new status. All he knows is that he's the one Wallfacer that Trisolaris wants dead.

In the midst of an interstellar war, Yun Tianming found himself on the front lines. Riddled with cancer, he chose to end his life, only to find himself flash frozen and launched into space where the Trisolaran First Fleet awaited. Captured and tortured beyond endurance for decades, Yun eventually succumbed to helping the aliens subjugate humanity in order to save Earth from complete destruction.

Set against the backdrop of China's Cultural Revolution, a secret military project sends signals into space to establish contact with aliens. An alien civilisation on the brink of destruction captures the signal and plans to invade Earth. Meanwhile, on Earth, different camps start forming, planning to either welcome the superior beings and help them or to fight against the invasion.

In those days, Earth was a planet in space. In those days, Beijing was a city on Earth. On this night, history as known to humanity came to an end. Eight light years away, a star has died, creating a supernova event that showers Earth in deadly levels of radiation. Within a year, everyone over the age of 13 will die. And so the countdown begins. Parents apprentice their children and try to pass on the knowledge they'll need to keep the world running. But the last generation may not want to carry the legacy of their parents' world....